The gunman, who was wearing a bullet-proof vest and dressed entirely in black, was not on any watch list that would have alerted authorities that he was dangerous, officials said. The incident was not believed to have any connection to international terrorism, they added.

The car Holmes was driving had Tennessee plates, but police sources told NBC News that they believed he had been living locally. Federal authorities told NBC News that Holmes has no military history.

Police said the gunman entered through an exit door and appeared at the front of the theater in Aurora, Colo., and released a canister which let out a hissing sound, which was thought to be tear gas.

He then started shooting into the crowd, sparking pandemonium.

In addition to those killed, at least 50 were wounded when the gunman opened fire at the screening of the blockbuster "The Dark Knight Rises," authorities and witnesses said.

At least 12 people were killed early Friday when at least one gunman opened fire at a midnight screening of the summer blockbuster "The Dark Knight Rises" in Aurora, Colo. NBC's Matt Lauer reports.

Witnesses told reporters that gunfire erupted during a shootout scene in the film.